Aluminum etching process

ABSTRACT

An etching process is provided for aluminum metal which produces a uniform surface with low reflectance and hides surface imperfections while at the same time increasing the whiteness of the surface. The etching process provides successive or multiple stage short-time etching treatments in which the first treatment employs special etching solutions based on alkali metal hydroxide, chelating agent and special oxidizing agents. The second or successive etching treatment employs conventional alkali metal hydroxide etching solutions. The multiple stage etching process produces aluminum metal suitable for architectural purposes without the necessity of using mechanical buffing prior to etching.

United States Patent [191 Smith ALUMINUM ETCHING PROCESS [75] Inventor: Harold Vernon Smith, Philadelphia,

[73] Assignee: Pennwalt Corporation, Philadelphia,

[22] Filed: Aug. 9, 1972 [211 App]. No.: 278,891

[451 Apr. 9, 1974 Primary Examiner-William A. Powell Attorney, Agent, or Firm-R0bert G. Danehower [5 7] ABSTRACT An etching process is provided for aluminum metal which produces a uniform surface with low reflectance and hides surface imperfections while at the same time increasing the whiteness of the surface. The etching process provides successive or multiple stage short-time etching treatments in which the first treatment employs special etching solutions based on alkali metal hydroxide, chelating agent and special oxidizing agents. The second or successive etching treatment employs conventional alkali metal hydroxide etching solutions. The multiple stage etching process produces aluminum metal suitable for architectural purposes without the necessity of using mechanical buffing prior to etching.

10 Claims, No Drawings Y ALUMINUM ETCHING PROCESS BACKGROUND OF THE INVENTION The appearance of many aluminum products is often marred by scratches, handling or machining marks, oxidation and corrosion pits. Many such products are subjected to an etching treatment which produces a more uniform pleasing appearance on the aluminum surface by removing or covering surface imperfections.

Aluminum finishers, using the conventional available etching processes, find that they may have to etch their aluminum products for as long as 15 minutes to achieve the desired improvement in appearance. Such prolonged treatments add considerably to the cost of aluminum metal finishing, in that large amounts of metal may be removed, and large amounts of etching chemicals may be consumed by the prolonged etching.

The conventional aluminum etching solutions generally employ aqueous alkali metal hydroxide solutions and a chelating agent. The conventional etching solution usually comprises a strong sodium hydroxide solution of about 15 to 250 grams per liter and a chelating agent to keep the dissolved aluminum from redepositing sodium aluminate on the metal surface. These etching solutions normally produce a silver to silver gray metallic appearance. In addition to the silver color such surfaces will have a high reflectivity and may be shiny. In addition the etched surface will be non-uniform in character and there will be no uniformly grained appearance. The resulting overall surface will not hide the die or extrusion marks and scratches are often visible.

The conventional etching solutions remove large amounts of aluminum metal from the surface without providing a good hiding or obscuring of the surface imperfections. The surface produced by a chelated caustic solution treatment gives an etched surface slightly more pleasing than the raw metal while hiding only a small amount of surface imperfections.

Highly reflecting surfaces may be obtained'by using a combination of high nitrate and low chlorite concentrations so as to produce a mirror surface in U.S. Pat. No. 2,673,143. Similar brightening effects were obtained in U.S. Pat. No. 2,671,717 by using high concentrations of nitrate.

As the demand for a better hide of surface imperfections increased a frosty etched finish was provided by various additions of phosphates and/or fluorides. This was an improvement over the chelated caustic etching, but did not gain wide acceptance because of insufficient hide of surface irregularities.

The next development in etching aluminum for architectural purposes such as for window and door framing and curtain walls was the use of chromates in etching solutions such as described in U.S. Pat. No. 3,314,890. This treatment provided frosty white matte etched finishes with good hide of surface imperfections. The chromate etching system also eliminated the problem of galvanizing.

The above etching processes removed as much as 5 percent by weight of the metal being etched because of the long etching cycles which ran from to minutes in contact with the caustic soda. This produces excessive chemical consumption of the etching solutions as well as creating serious disposal problems of the spent solutions containing tremendous amounts of dissolved aluminum metal.

To meet the ever-increasing demand for architectural aluminum which is non-reflecting and at the same time free of surface imperfections, the aluminum processor first mechanically buffed the aluminum to remove scratches, pits, die lines, rolling marks or any surface irregularity. This mechanical buffing left the alu-- SUMMARY OF INVENTION.

I have now discovered a process .for etching unbuffed aluminum metal with alkali metal hydroxide solutions which produces architectural aluminum characterized by having a surface frosty and white in color and free of irregularities and other imperfections. The etched surface is uniform in appearance with low reflectance and, under preferred etching conditions, the surface is non-image forming. This highly desirable finish is obtained without the necessity of mechanical buffing.

My improved etching process for unbuffed aluminum metal comprises successive short-time etching treatments employing as the etchant aqueous alkali metal hydroxide solutions. My invention is also characterized in that the first etching treatment takes place in alkali metal hydroxide solutions containing one or more particular oxidizing agents. The second etching treatment takes place in conventional alkali metal hydroxide etching solutions.

Generally, in my etching process only a first etch with the alkali solution containing the oxidizer and a second etch in theconventional etching solution are all the etching treatments that are required to produce frosty white low reflecting aluminum surfaces. However, additional etching cycles may be employed if necessary. The total etching time of a two etch-cycle is generally less'than the time employed in a single etching of aluminum metal which'has been mechanically buffed before etching.

The first etching solution comprises one or more oxidizing agents in solution with the alkali metal hydroxide. The second or successive etching treatment usually employs the conventional aqueous alkali metal hydroxide etching solution and need not contain the oxidizing agent of the first etching solution for reasons of economy. However, from the standpoint vof surface obtained, the second etching solution may also contain one or more of the selected oxidizing agents.

The oxidizing agent which is'employed in the first etching solution is selected from the group consisting of persulfates, peroxides, perborates, periodates, chlorites, chlorates, perchlorates, iodates, hypochlorites, nitrites, peroxyphosphates, and bromates. The oxidizing agent is used at concentrations of about 2 to about grams per liter. The alkali metal hydroxide concentration will be at about 15 to about 50 grams per liter.

Special chelating agents are required for use in the etching solution containing the oxidizing agent and consist of one or more compounds selected from the group consisting of sorbitol, gluconic acid and glucoheptonic acid and their alkali metal salts.

The first etching process using the oxidizing agent is operated at a temperature of about 130 to about 200F. The preferred operating range is about 150 to about 170F. In general, the higher the operating temperature, the faster is the rate of etching. The second or successive etching using the conventional etching solution is operated within the range of about 120 to about 190F.

Between the successive etching treatments the aluminum is rinsed with water and then desmutted if necessary as shown in U.S. Pat. No. 3,275,562 to remove surface impurities not attacked by the caustic soda. If desmutting is unnecessary, the aluminum work piece goes directly to the second etching treatment following a water rinse. The improved etching process may comprise successive'or multiple stage etching treatments for aluminum such as contacting unbuffed aluminum with (l) etching solution containing oxidizing agent, (2) a conventional etching solution, (3) an etching solution containing oxidizing agent and finally, (4) a conventional etching solution.

Alternatively, the etching cycle may consist of (l) etching in a solution containing oxidizing agent, (2) etching in conventional etching solution, and (3) etching in conventional etching solution.

After the second or final etching treatment, the work piece is rinsed with water, desmutted and rinsed again with water. Thereafter, it may go to an anodizing process or to paint preparation processes.

Both the etching solution containing the oxidizer of the first etching stage and the conventional alkali metal hydroxide etching solution of the second or successive stage may contain other additives such as aluminum surface modifying agents. These may be surfactants such as fluoroalkyl surfactants as shown in U.S. Pat. No. 3,557,000 which provide long-lasting bath life. As used hereafter in the specification and claims the term fluoroalkyl surfactant means a fluorine containing surfactant as disclosed and claimed in U.S. Pat. No. 3,557,000. Special surfactants to improve rinsing and to reduce fuming and foaming, fluorides and phosphates which produce special. surface effects on the aluminum as shown in U.S. Pat. No. 2,653,861 may be used.

DETAILED DESCRIPTION OF THE INVENTIGN My new etching treatment is generally employed on the aluminum alloys used for architectural purposes. These alloys are generally 6061, 6062, 6063, 6463, and KB45 for extrusion purposes, and 3003, 5005, 5052, 6061 for sheet etching. The process is equally satisfactory on other aluminum alloys not generally used for architectural purposes such as high strength corrosion resistant alloys used in automobile and aircraft industry. Examples are 2024 and 7075.

The aluminum metal should be carefully prepared for etching by first removing any cutting, rolling or stamping oils and greases. This is usually done by cleaning 6 The concentration of the alkali in the first or oxidizing etching solution is within the range of about 15 to about 50 grams per liter expressed as sodium hydroxide. Higher concentrations of alkali causes rapid depletion of the oxidizing agent. A concentration of 30 to 45 grams/liter is preferred for optimum efficiency.

In the second or conventional etching solution, the caustic concentration is within the range of about 15 to about 200 grams per liter expressed as sodium hydroxide.

The etching processes of my invention are usually performed by immersion of the work pieces in the etching solution contained in a steel tank. The tanks are equipped with coils for heating and/or cooling the etching bath. Agitation of the bath is desirable to maintain a uniform concentration of the etching solution, particularly at the interface with the work piece. Good ventilation is required to dispel the caustic fumes and byproduct hydrogen gas.

My process for etching aluminum using successive or multiple stage etching treatments is applicable to immersion or spray operation. Immersion is most generally used.

Generally, the first etching treatment containing the oxidizer is continued until a frosty, white appearing surface is obtained and there is a leveling of surface irregularities. The second etching treatment is continued until the aluminum surface has a uniform appearance and which has a low-reflectance. At this time nearly all surface irregularities will have been removed and the aluminum surface is frosty and white. The time for each etching treatment will be about l to 4 minutes in duration. However, etching treatments lasting from 1 up to 6 minutes have been used. When two successive etching treatments of 2 minutes each are employed in my process this 4 minutes of etching time compares with an etching time of 10 to 15 minutes using the convention etching processes and solutions on aluminum metal which has previously been mechanically buffed.

In commerical practice of my successive treatment etching process, after a satisfactory surface condition is obtained, it is desirable to reduce the etching time by small increments in the first etching bath and observe the results of each reduction of time on the appearance of the metal surface. The etching time is reduced in the first etching bath in this manner until a surface appearance is obtained following the second etching which is barely acceptable. This would then be the minimum time for immersion in the first etching solution. A minimum immersion time is desirable in the first etching solution which contains the oxidizer since this is the most expensive etching solution to replenish.

The sources of oxidizing agents used in the first or oxidizing etching solution for etching unbuffed aluminum are as follows:

for persulfates ammonium 'persulfate, sodium persulfate and potassium persulfate; for peroxides hydrogen peroxide and sodium peroxide;

for perborates sodium perborate and potassium perborate; for periodates sodium periodate and potassium periodate; for chlorites sodium chlorite and potassium chlorite;

for chlorates sodium chlorate, potassium chlorate and ammonium chlorate;

for perchlorates sodium perchlorate and potassium perchlorate;

for iodates sodium iodate and potassium iodate;

for hypochlorites sodium hypochlorite, potassium hypochlorite and calcium hypochlorite;

for nitrites sodium nitrite, potassium nitrite and ammonium nitrite;

for bromates potassium bromate; and

for peroxyphosphates potassium peroxydiphosphate.

The oxidizing agent in the alkali metal hydroxide solution is used at a concentration of about 2 to 120 grams/liter. About 2 grams/liter is the minimum con centration of oxidizing agent which will provide acceptable surface appearance. This is observed by visual inspection and/or by measuring both total and diffuse reflectivity with a Photo-Volt Reflectometer. A preferred concentration range for the oxidizing agent is about 30 to 60 grams per liter. At this concentration level the reflectance of the etched aluminum is generally nonimage forming. At concentrations higher than 120 grams/liter no increased benefits are obtained.

When nitrites are employed in the first etching solution it is desirable but not necessary to have nitrate present at about 5 percent by weight of the nitrite content. The nitrate is supplied as one of the compounds selected from the group consisting of sodium nitrate, potassium nitrate and ammonium nitrate. A preferred concentration of alkali metal nitrites is 5 to 25 grams per liter of water.

The oxidizing agent may be supplied to the bath in the form of a powder or as an aqueous solution.

In operation of the etching baths it is desirable to analyze the first or oxidizing etching solution every two hours for concentration of the oxidizing agent and concentration of the caustic alkali. Replenishments are made as indicated by the analyses. The chelating agent is conveniently formulated and supplied in combination with the caustic alkali and no separate analysis for this agent is necessary.

The second or conventional etching solution should be analyzed for caustic etchant at least every four hours and replenishment of etchant should be made as indicated by the analyses. In this etching solution, the chelating agent is conveniently supplied in combination with the caustic.

The chelating agents which are efficient and compatible in the first etching solution containing the oxidizer are sorbitol. gluconic acid and glucoheptonic acid and their alkali metal salts. These chelating agents are used at concentrations well known in the art and are generally within the range of 0.5 to grams/liter. As a general rule, the minimum amount of chelating agent may be calculated as 2 percent by weight of the alkali metal hydroxide content.

In the conventional or non-oxidizing etching solution the chelating agent may be any agent useful for suppressing alumina precipitation during etching. Among such agents are sorbitol, gluconic acid, glucoheptonic acid, mannitol, ascorbic acid, sorbose, tannic acid, ethylenediamine tetraacetic acid, sodium chrom glucosate, diglycolic acid, picolinic acid, aspartic acid, dithiooxamide, d-glyconolactone, and l-rhamnose. These chelating agents are used at concentrations well known in the art and are generally within the range of 0.5 to 15 grams/liter. As a general rule, the minimum amount of chelating agent may be calculatedas 2 percent of the alkali metal hydroxide content.

The best mode of practicing my invention will be apparent from a consideration of the following examples: In these experiments various etching sequences were employed varying the composition of the etchant solution and the duration of the treatment. The etching results were measured with a PhotoVolt Reflectometer, Model 610. Total reflectivity was measured as a percent of the reflectivity obtained by setting the reflectometer at on the scale using cleaned, deoxidized but unetched aluminum metal. A total-reflectivity reading of 70 would represent a 30 percent decrease in total reflectivity.

Diffuse reflectivity was measured by setting the diffusion head of the PhotoVolt Reflectometer at 25 on the scale when reading the reflection from cleaned, deoxidized but unetched aluminum metal. At this reading the surface of the aluminum appears slightly frosty, slightly white with poor hide. of surface imperfections. Any reading about 25 represented an increase in diffuse reflectivity. This increase would correspond to an increase in the number of corrosion pits in the aluminum metal caused by the etching process. A diffuse reflectivity of 30 represents a minimum level of diffuse reflectivity which corresponds to an acceptable level v for use of the aluminum for architectural purposes. At adiffuse reflectivity of 30 there is good hide of surface imperfections and the aluminum appears uniform, frosty and moderately white in color. As used throughout this specification and claims the expression low reflectance means that the metal surface has a diffuse reflectivity of 30 or higher as measured in the manner set forth above. Any diffuse reflectivity reading higher than 30 also represents a satisfactory etch. At a diffuse reflectivity of about 45 virtually all surface imperfections have been hidden and the aluminum surface appears frosty and uniformly white.

In allof the experiments the aluminum metal was prepared for etching by cleaning in an inhibited-mild alkaline cleaner, rinsed'with with water, deoxidized in an inhibited acid deoxidizer, again rinsed with water and then etched. The same aluminum alloy and the same chelating agent were used to'assure comparative results. After etching, the metal was rinsed with water, desmutted in an inhibited acid desmutter, rinsed again with water and dried in air. The etched surface was then immediately examined visually and with the reflectometer.

EXPERIMENT 1 This experiment represents a conventional etching treatment with a conventional etching solution.

Aluminum extrusions were etched for 8 minutes at F in an aqueous solution containing 29.4 grams/- liter sodium hydroxide and 0.6 grams/liter of sorbitol. The total reflectivity measured 69.9 and the diffuse reflectivity was 25. The surface appeared slightly frosty and slightly white. There was poor hide of metal imper- EXPERIMENT 2 This experiment discloses the effect of multiple stage or successive short time etching treatments using conventional alkali metal hydroxide etching solutions in both stages.

Aluminum extrusions were first etched in an aqueous solution containing 29.4 grams/liter sodium hydroxide and 0.6 grams/liter of sorbitol for 4 minutes at 160F. The extrusions were rinsed with water, desmutted and then contacted with a second etching solution by immersion containing 29.4 grams/liter sodium hydroxide and 0.6 grams/liter of sorbitol for 4 minutes at 160F.

The total reflectivity measured 60.1 and the diffuse reflectivity was 27.5. The metal appeared slightly frosty, whiter than in Experiment 1 with fair hide of surface imperfections. This etching treatment was not satisfactory for producing aluminum for architectural purposes.

EXPERIMENT 3 This experiment represents a conventional etching treatment in which the etching solution was modified by inclusion of sodium chromate and a surfactant which provides more efficient alkali consumption and extends the bath life.

Aluminum extrusions were immersed in an aqueous solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant as disclosed in U.S. Pat. No. 3,557,000 and 0.0525 grams/liter of sodium chromate for 8 minutes at a temperature of 160F.

The surface measured a total reflectivity of 25.5 and a diffuse reflectivity of 30.0. The surface appeared frosty, moderately white with fair hide of surface imperfections. The etched surface was considered borderline for architectural uses.

EXPERIMENT 4 This experiment shows the results of two 4 minute etches with an in-between water rinsing, desmutting and another water rinse as compared to a single 8 mintemperature was 160F for each etching.

After the two stage etching treatment the surface of the aluminum measured 20.0 total reflectivity and 32.5 diffuse reflectivity. The surface appeared frosty, slightly whiter than in Experiment 3 with good hide of surface irregularities. This surface was barely satisfactory for architectural uses.

EXPERIMENT ute etch in the solutions used in Experiment 3. The

bitol, 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate for 4 minutes at a temperature of F.

After rinsing and drying the total reflectivity measured less than 1.0 and the diffuse reflectivity measured 45.0. The surface appeared frosty, white with very good hide of surface imperfections. No image was observable in the reflection. The etched surface was excellent for architectural uses.

EXPERIMENT 6 In this test sodium gluconate was used as the chelating agent. Aluminum extrusions were first immersed in an etching solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sodium gluconate, 45 grams/liter of sodium nitrite and 5.0 grams/liter of sodium nitrate for 4 minutes at 160F.

The aluminum metal was then etched in a second solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate for 4 minutes at 160F.

The total reflectivity was 9.0 and the diffuse reflectivity measured 38. The metal surface appeared frosty and white and had very good hide.

EXPERIMENT 7 In this test, sodium glucoheptonate was substituted for the sodium gluconate of Experiment 6 with the following results:

The total reflectivity measured 4 and the diffuse reflectivity measured 44. The metal surface appeared frosty and very white with excellent hide of surface irregularities.

EXPERIMENT 8 This experiment shows the minimum concentration level of oxidizing agent to obtain a diffuse reflection of 30 which is considered the minimum level of acceptability for architectural purposes.

Each oxidizing agent was tested in a sodium hydroxide solution at 29.4 grams/liter and 0.6 grams/liter of sorbitol at a temperature of 160F for 4 minutes. The second etching solution contained 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate. Immersion of the work piece was 4 minutes at 160F.

The concentration of the oxidizing agent in thefirst etching solution was varied until a diffuse reflectivity reading of 30 was obtained after the two etch treatment. The resultsappear in Table l. r

Ammonium pet-sulfate 3.0 Sodium chlorate 2. Potassium peroxydiphosphate Hydrogen peroxide Potassium periodote Sodium chlorite Calcium hypochlorite Potassium bromate Sodium perborate EXPERIMENT 9 TABLE III Grams of Oxidizer Consumed Per Gram In this ser es of teststhe upper concentration range 1 of Aluminum of the oxidizing agents 11'] the first etching bath was ex- 5 Bath Composition Removed by Etching ed. In each test the aluminum work iece was sub- P p o Sodium Nitrite 20.0 grams/liter 5.3 ected to a second etching for 4 minutes at 160 Fm the sodium Nitrate 1'0 grams/lite, second etching bath described in Experiment 8. Table g i Hydroxide 3-; s j or itol grams iter 11 shows the composition of the first or oxidizing etch- Sodium Nitrite 200 grams/liter 21.0 ing solution and the observations made on the metal Sodium Nitrate 1.0 grams/liter Sodium Hydroxide 92.2 grams/liter surface after the second etching. Sorbitol L8 grams/m Sodium Nitrite 20.0 grams/liter 9.5 Sodium Nitrate 1.0 grams/liter Sodium Hydroxide 41.75 grams/liter Sorbitol 0.83 grams/liter 15 Sodium Nitrite 20.0 grams/liter 13.5 EXPERIMENT 10 Sodium Nitrate 1.0 grams/liter Sodium Hydroxide 60.0 grams/liter Sorbitol 1.2 grams/liter Sodium Nitrite 20.0-grams/liter 3.6 Sodium Nitrate 1.0 grams/liter This experiment 1nd1cates the operating range of al- Sodium Hydroxide 15.0 grams/liter kali metal hydroxide concentration in the first etching 2O Smbm" grams/m TABLE ll Composition First Bath Surface Observations Sodium nitrite 45.0 grams/liter Total reflectivity less than 1 Sodium hydroxide 29.4 grams/liter Diffuse reflectivity 41 Sorbitol 0.6 grams/liter Frosty. very white. very good hide. Hydrogen peroxide 45.0 grams/liter Total reflectivity 23 Sodium hydroxide 29.4 grams/liter Diffuse reflectivity 41 Sorbitol 0.6 grams/liter Frosty. very white. very good hide. Sodium chlorite 45.0 grams/liter Total reflectivity 32 Sodium hydroxide 29.4 grams/liter Diffuse reflectivity 38.5 Sorbitol 0.6 grams/liter Slightly frosty. white. good hide. Calcium hypochlorite 45.0 grams/liter Total reflectivity 58 Sodium hydroxide 29.4 grams/liter Diffuse reflectivity 36 Sorbitol 0.6 grams/liter Slightly frosty. slightly white.

fair hide. Sodium Nitrite 80.0 grams/liter Total reflectivity 13.5 Sodium Hydroxide 29.4 grams/liter Diffuse reflectivity 49 Sorbitol 0.6 grams/liter Frosty. white. excellent hide. Sodium nitrite 1200 grams/liter Total reflectivity 13.0 Sodium hydroxide 29.4 grams/liter Diffuse reflectivity 49.0 Sorbitol 0.6 grams/liter Frosty. very white. excellent hide Sodium chlorate 120.0 grams/liter Total reflectivity 54.0 Sodium hydroxide 29.4 grams/liter Diffuse reflectivity 28.5 Sorbitol 0.6 grams/liter Frosty, white. fair hide. Sodium perborate 100.0 grams/liter Total reflectivity 24.5 Sodium hydroxide 29.4 grams/liter Diffuse Reflectivity 37.0 Sorbitol 0.6 grams/liter Frosty. white. uniform.

good hide. Potassium peroxydiphos- 120.0 grams/liter Total reflectivity 51.0 phate Sodium hydroxide 29.4 grams/liter Diffuse Reflectivity 33.5 Sorbitol 0.6 grams/liter Slightly frosty. White. uniform.

fair to good hide.

bath which contains the oxidizing agent. EXPERIMENT 1 l The concentration of the caustic soda was varied in 60 These tests show operabletemperatures in the first the first etching bath while the concentration of the oxetching bath containing the oxidizer. The aluminum idizer was identical (20 grams/liter) at the start up of was etched for 4 minutes in the first bath at varying each test. The consumption of the oxidizer was meaconcentrations and temperatures followed by a second Y o sured after 4 minutes of etching, at 160F. At concenetching, after water rinsing, for 4 minutes at 160 F in trations of 60.0 grams/liter and higher of sodium hyabath containing 29.4 grams/liter of sodium hydroxide. droxide the consumption of oxidizing agent was exces- 0.6 grams/liter of sorbitol. 0.0015 grams/liter of fluosive for economic operation. The results appear in roalkyl surfactant and 0.0525 grams/liter of sodium Table III. chromate. The results are shown in Table lV.

TABLE IV lst Etching Solution Containing Oxidizer Metal Surfaces Composition Temperature F Total Reflectivity 32.0 Diffuse Reflectivity 38.5

i Slightly frosty, white,

good hide, uniform. Total Reflectivity 39.5 Diffuse Reflectivity 33.5

Frosty, moderately white.

uniform, fair to good Total reflectivity 56.5 Diffuse reflectivity 30.0 Slightly frosty, white, fair to good hide.

Total reflectivity 31.0 Diffuse reflectivity 37.5 Frosty, white, uniform,

Total reflectivity 49.0 Diffuse reflectivity 3L0 Slightly frosty, white, fair to good hide. Frosty, very white, uniform, excellent hide. non-image forming Sodium hydroxide Sorbitol 29.4 grams/liter 0.6 grams/liter Total reflectivity less than 1 Diffuse reflectivity 34.0

Frosty, white, good hide,

uniform.

EXPERIMENT 12 This experiment shows varying levels of concentration for the chelating compound in the first etching solution containing the oxidizer. Apart from the chelating compound, the first etching solution contained 45.0 grams per liter of sodium chlorite and 29.4 grams per liter of sodium hydroxide. The first etching by immersion was for 4 minutes at 160F. After a water rinse, desmutting and second water rinse, the work pieces were etched for 4 minutes at l60F in the second etching solution described in Experiment H. The results appear in Table V.

TABLE V Composition of lst Etching Solution Metal Surfaces Sodium chlorite 45.0 grams/liter Total reflectivity less than 1 Sodium hydroxide Sorbitol 29.4 grams/liter 5.0 grams/liter Frosty, white, uniform.

good hide.

Total reflectivity 2.0 Diffuse reflectivity 32.5 Frosty, white, fair to good hide, uniform.

45.0 grams/liter I 29.4 grams/liter 10.0 grams/liter Sodium chlorite Sodium hydroxide Sorbitol Diffuse reflectivity 36.0

etched surfaces having a uniform appearance, the improvement comprising subjecting unbufied aluminum metal to successive short-time etching treatments in which the unbuffed aluminum metal is contacted with a first aqueous alkali metal hydroxide etching solution comprising the following composition expressed in grams per liter of water:

Alkali metal hydroxide 15 to 50, at least one chclating agent selected from the group consisting of sorhitol, gluconic acid, glucoheptunic acid. and their alkali metal salts and at least one oxidizing compound selected from the group consisting of nitrites, pcrsulfatcs, chlorites, pcroxyphosphates, chlorates, perchlorates, peroxides, iodates, hypochlorites, perborates, pcriodatesand bromates 2 to l20,

at a temperature within the range of about 130 to about 200F, for a time sufficient to produce a frosty. white appearing surface, and thereafter the aluminum metal is rinsed with water, and then contacted with a second aqueous alkali metal hydroxide etching solution at a temperature within the range of 120 to 1'90F for a time sufficient to produce a surface on the aluminum metal which is uniform in appearance and with low- Sodium chlorite 45.0 grams/liter Total reflectivity 3.5 Sodium hydroxide 29.4 grams/liter Diffuse reflectivity 30.0 reflectance d fi ll h l i metal i i d Sorbitol 15.0 grams/liter Frosty, white, fair hide.

. with water.

2. The process of claim 1 in which the second aqueous alkali metal hydroxide etching solution comprises the following composition expressed in grams per liter 1 claim: of water:

1. In the process of etching aluminum metal in alkali Alkali mmi hydroxide 15 w 200 metal hydroxide solutions to produce low-reflecting d s agent 05 10 3. The process of claim 2 in which the second aqueous alkali metal hydroxide etching solution contains at least 0.0002 percent by weight of a fluoroalkyl surfactant.

Alkali metal hydroxide 15 to 50 Chelating agent selected from the group consisting of sorbitol, gluconic acid, glucoheptonic acid, and their alkali metal salts 0.5 to l5 Alkali metal nitrite 5 to 25 Alkali metal nitrate l to 5 S. The process of claim 1 in chelating agent.

which the first alkali metal hydroxide etching solution contains sorbitol as its 6. The process for etching aluminum which comprises subjecting the aluminum work piece to at least two successive etching treatments as provided by claim 1. 7. The process of claim 1 in which the oxidizing agent in the first etching solution is selected from the group consisting of sodium, potassium and ammonium nitrite, and the concentration of the nitrite compound is 5 to grams/liter.

8. The process of claim 7 in which the first etching solution also contains a nitrate compound in an amount equivalent to 5 percent by weight of the nitrite compound.

9. The process of claim 8 in which the nitrate compound is selected from the group consisting of sodium, potassium, and ammonium nitrate.

10. The process of claim 1 in which the aluminum metal is desmutted between the first and second etching treatments. 

2. The process of claim 1 in which the second aqueous alkali metal hydroxide etching solution comprises the following composition expressed in grams per liter of water:
 3. The process of claim 2 in which the second aqueous alkali metal hydroxide etching solution contains at least 0.0002 percent by weight of a fluoroalkyl surfactant.
 4. The process of claim 1 in which the first aqueous alkali metal hydroxide aluminum etching solution comprises the following composition expressed in grams per liter of water:
 5. The process of claim 1 in which the first alkali metal hydroxide etching solution contains sorbitol as its chelating agent.
 6. The process for etching aluminum which comprises subjecting the aluminum work piece to at least two successive etching treatments as provided by claim
 1. 7. The process of claim 1 in which the oxidizing agent in the first etching solution is selected from the group consisting of sodium, potassium and ammonium nitrite, and the concentration of the nitrite compound is 5 to 120 grams/liter.
 8. The process of claim 7 in which the first etching solution also contains a nitrate compound in an amount equivalent to 5 percent by weight of the nitrite compound.
 9. The process of claim 8 in which the nitrate compound is selected from the group consisting of sodium, potassium, and ammonium nitrate.
 10. The process of claim 1 in which the aluminum metal is desmutted between the first and second etching treatments. 